Why is Sheldon Scared of Birds ?

The Big Bang Theory episode 09 of the 5th season titled as "The Ornithophobia Diffusion" is already aired on CBS. It was quite funny. A bird came to Sheldon's window and he became scared like crazy! In his usual way he knocked at the window two times and said " Bird! Go away! Bird ! Go away ! Bird ! Go away !"  but it did not !

Later he explained Raj why he is so scared of birds. In Sheldon's way -

• Age 7, a blood thirsty chicken chases me up a tree !
• Age 12, a Mag Pie tries to seal the retainer out of my mouth !
• Age 16, a Parrot in a pet store calls me fat ass !

Bernadette (Melissa Rauch, right) tries to help Sheldon (Jim Parsons, left) overcome his fear of birds, on THE BIG BANG THEORY. Photo: Michael Yarish/Warner Bros.
 courtesy: CBS